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CHSE Odisha Voc. Microbiology & Serology study materials
CHSE Odisha Voc. Microbiology & Serology study resources

CHSE Odisha Voc. Microbiology & Serology practice questions

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Question 1Introduction to Microbiology and Laboratory Safety

Which instrument is used to sterilise culture media and glassware by exposing them to steam at 121 °C and 15 psi for about 15 minutes?

  1. Autoclave Correct answer.
  2. Hot air oven
  3. Centrifuge
  4. Laminar air flow cabinet

Explanation

An autoclave uses saturated steam under pressure, which raises the temperature above the boiling point of water so that moist heat kills endospores and vegetative cells. Hot air ovens use dry heat, while laminar flow cabinets and centrifuges do not sterilise materials.

Question 2Introduction to Microbiology and Laboratory Safety

A laboratory technician spills a small volume of bacterial culture on the bench. Which action should be taken first?

  1. Flood the area with an appropriate disinfectant and allow contact time Correct answer.
  2. Report the spill to the supplier of the culture
  3. Ignite the spill with a spirit lamp to burn the organisms
  4. Wipe the spill towards the bench edge with a dry cloth

Explanation

Spill management begins by covering or flooding the contaminated area with a suitable disinfectant and giving it the manufacturer's contact time so the organisms are inactivated before cleaning. Dry wiping or flaming spreads or aerosolises the organisms, and supplier notification is irrelevant to immediate containment.

Question 3Introduction to Microbiology and Laboratory Safety

Which group of microorganisms is defined as acellular infectious agents that replicate only inside a living host cell?

  1. Bacteria
  2. Protozoa
  3. Viruses Correct answer.
  4. Fungi

Explanation

Viruses lack independent metabolism and must use a host cell's machinery to replicate, which is why they are described as obligate intracellular parasites and are not cells themselves. Bacteria, fungi and protozoa are cellular organisms capable of their own metabolic activity.

Question 4Introduction to Microbiology and Laboratory Safety

In the Gram staining procedure, what is the purpose of applying iodine after the crystal violet step?

  1. It acts as a mordant that fixes the crystal violet inside the cell Correct answer.
  2. It counterstains the Gram-negative cells pink
  3. It removes excess crystal violet from the slide
  4. It decolourises the cell wall of Gram-negative bacteria

Explanation

Iodine combines with crystal violet to form a large crystal violet-iodine complex that is trapped in the thick peptidoglycan of Gram-positive cells. Decolourisation is done with alcohol or acetone, and the pink counterstain is safranin applied later.

Question 5Introduction to Microbiology and Laboratory Safety

Which class of biological safety cabinet provides protection to both the operator and the sample by maintaining a partial barrier with HEPA-filtered air and is commonly used for handling clinical specimens?

  1. Class I cabinet
  2. Horizontal laminar flow clean bench
  3. Class III cabinet
  4. Class II cabinet Correct answer.

Explanation

Class II cabinets are the standard for clinical microbiology because they protect the operator, the environment and the product through HEPA-filtered downflow and exhaust air. Class I protects only the operator and environment, Class III is a fully sealed glove-box, and a clean bench protects only the work.

Question 6Introduction to Microbiology and Laboratory Safety

What does the term 'in vitro' mean when applied to microbiological culture work?

  1. Performed in an artificial laboratory environment outside a living organism Correct answer.
  2. Performed only in a hospital ward
  3. Performed within a living animal
  4. Performed without any nutrients

Explanation

In vitro literally means 'in glass' and refers to experiments or cultures carried out in artificial laboratory systems such as tubes and plates. In vivo refers to work within a living organism.

Question 7Introduction to Microbiology and Laboratory Safety

Which of the following is the correct way to dispose of used sharps such as needles and scalpel blades in a microbiology laboratory?

  1. Recap the needle and place it in a general waste bin
  2. Wrap it in paper and hand it to the housekeeping staff
  3. Place it directly into a puncture-resistant sharps container without recapping Correct answer.
  4. Store it on the bench for reuse after flaming

Explanation

Sharps must be discarded immediately into a designated puncture-resistant, leak-proof sharps container without recapping, since recapping is a leading cause of needlestick injury. General waste and staff handling expose people to bloodborne pathogens.

Question 8Introduction to Microbiology and Laboratory Safety

A student observes a smear that appears blue-violet under the oil immersion lens after Gram staining. What is the most reasonable interpretation?

  1. The cells are Gram-negative bacilli
  2. The cells retained the crystal violet-iodine complex and are Gram-positive Correct answer.
  3. The student used safranin as the primary stain
  4. The smear was not heat-fixed before staining

Explanation

Blue-violet colour after Gram staining indicates that the crystal violet-iodine complex was retained during decolourisation, which is characteristic of Gram-positive cells. Gram-negative cells lose the primary stain and take up the pink safranin counterstain.

Question 9Introduction to Microbiology and Laboratory Safety

Which statement best describes the role of a HEPA filter in a biological safety cabinet?

  1. It neutralises chemical fumes by absorption
  2. It kills microorganisms by ultraviolet irradiation
  3. It maintains a constant temperature inside the cabinet
  4. It removes particles of about 0.3 micrometres with at least 99.97 percent efficiency Correct answer.

Explanation

A HEPA filter physically traps aerosols and particles with a rated efficiency of at least 99.97 percent at the most penetrating particle size near 0.3 micrometres. It does not kill organisms, control temperature, or absorb gases.

Question 10Introduction to Microbiology and Laboratory Safety

Which of the following is a defining feature that distinguishes bacteria from fungi?

  1. Bacteria possess a nucleus bounded by a membrane
  2. Bacteria are prokaryotes lacking a membrane-bound nucleus Correct answer.
  3. Fungi lack a cell wall
  4. Fungi are always smaller than bacteria

Explanation

Bacteria are prokaryotes, meaning their genetic material is not enclosed in a membrane-bound nucleus, whereas fungi are eukaryotes with a true nucleus. Most fungi also possess a chitin-containing cell wall, and size is not a reliable distinguishing criterion.

Question 11Introduction to Microbiology and Laboratory Safety

A laboratory worker needs to sterilise a heat-sensitive plastic pipette tip and a loop of platinum wire. Which pairing of methods is most appropriate?

  1. Autoclaving for the plastic tip and dry heat for the wire loop
  2. Boiling water for the plastic tip and autoclaving for the wire loop
  3. Dry heat for the plastic tip and boiling for the wire loop
  4. Ethylene oxide or gamma irradiation for the plastic tip and flaming for the wire loop Correct answer.

Explanation

Heat-sensitive disposable plastics are sterilised by low-temperature methods such as ethylene oxide gas or gamma irradiation, while a platinum loop can be sterilised rapidly by flaming in a spirit lamp flame. Autoclaving or dry heat would deform the plastic.

Question 12Introduction to Microbiology and Laboratory Safety

What is the primary purpose of subculturing a bacterial isolate onto fresh medium in the laboratory?

  1. To increase the virulence of the organism
  2. To sterilise the culture vessel
  3. To change the Gram reaction of the organism
  4. To maintain viability and obtain fresh actively growing cells Correct answer.

Explanation

Subculturing transfers organisms to fresh nutrients so that they remain viable and continue active growth, preventing death from nutrient depletion and waste accumulation. It does not alter virulence or Gram reaction and is performed with already sterile vessels.

Question 13Introduction to Microbiology and Laboratory Safety

Which personal protective equipment is essential when handling an open tube of a suspected tuberculosis sputum sample in a Class II cabinet?

  1. Gloves, a laboratory coat and a fitted N95 or equivalent respirator Correct answer.
  2. Reusable cloth gloves without a gown
  3. Cotton gloves and open sandals
  4. A face shield alone without gloves

Explanation

Mycobacterium tuberculosis is transmitted by the airborne route, so a fitted particulate respirator in addition to gloves and a gown is required even inside a cabinet. Open footwear and inadequate gloves leave skin and feet exposed to contamination.

Question 14Introduction to Microbiology and Laboratory Safety

Which of the following best explains why immersion oil is used with the 100x objective lens?

  1. It has a refractive index similar to glass and reduces light loss, improving resolution Correct answer.
  2. It increases the magnification of the objective
  3. It prevents the lens from overheating
  4. It kills microorganisms on the slide

Explanation

Immersion oil has a refractive index close to that of glass, so light rays pass from the slide into the objective without bending away, increasing the numerical aperture and thus the resolving power. It does not increase magnification, cool the lens or disinfect the slide.

Question 15Introduction to Microbiology and Laboratory Safety

Which agency classification scheme groups laboratory agents into Risk Groups 1 to 4 based on the hazard they pose to workers and the community?

  1. Hazard analysis critical control point system
  2. ISO 9001 quality standard
  3. Pharmacopoeia monograph system
  4. World Health Organization laboratory biosafety guidance Correct answer.

Explanation

WHO laboratory biosafety guidance classifies infectious agents into Risk Groups 1 to 4 according to individual and community risk, and links each group to recommended containment levels. Pharmacopoeias, ISO 9001 and HACCP address drug standards, quality management and food safety respectively.

Question 16Introduction to Microbiology and Laboratory Safety

Which of the following is the correct sequence of steps for hand hygiene before leaving a microbiology laboratory?

  1. Wash gloves with disinfectant and keep them on outside the laboratory
  2. Apply alcohol rub over gloves and leave the laboratory
  3. Remove gloves, wash with soap and water or use alcohol rub, then dry hands Correct answer.
  4. Rinse with water only, then dry with a shared towel

Explanation

Gloves are removed first because they are contaminated, then hands are washed with soap and water or disinfected with an alcohol-based rub and dried. Gloves must never be worn outside the laboratory, and shared towels spread contamination.

Question 17Introduction to Microbiology and Laboratory Safety

A culture of a Risk Group 3 organism must be handled in a laboratory that meets which minimum containment requirement?

  1. Biosafety Level 2 with an open bench only
  2. Biosafety Level 1 with standard precautions only
  3. Biosafety Level 3 with controlled access and directional airflow Correct answer.
  4. Open bench with no special ventilation

Explanation

Risk Group 3 agents require Biosafety Level 3 containment, which includes controlled access, directional airflow and primary containment devices such as a Class II cabinet. Levels 1 and 2 are insufficient for such agents.

Question 18Introduction to Microbiology and Laboratory Safety

What is the main reason for heat-fixing a bacterial smear before staining?

  1. To kill the bacteria and help them adhere to the slide Correct answer.
  2. To swell the cells so they appear larger
  3. To provide nutrients for bacterial growth
  4. To remove the cell wall for staining

Explanation

Gentle heating coagulates bacterial proteins, killing the organisms and causing them to adhere to the glass so they are not washed off during staining. It does not enlarge cells, strip cell walls or supply nutrients.

Question 19Introduction to Microbiology and Laboratory Safety

Which of the following correctly describes the difference between disinfection and sterilisation?

  1. Sterilisation kills only vegetative cells, while disinfection kills endospores
  2. Disinfection destroys all microorganisms including endospores, while sterilisation removes only dirt
  3. Disinfection reduces or eliminates pathogenic organisms on inanimate surfaces but may not destroy all endospores, while sterilisation destroys all microbial life Correct answer.
  4. Both terms mean the same process and can be used interchangeably

Explanation

Disinfection lowers the number of pathogens on surfaces but does not necessarily inactivate resistant endospores, whereas sterilisation is the complete destruction or removal of all microbial life including endospores. The terms are therefore not interchangeable.

Question 20Introduction to Microbiology and Laboratory Safety

Which of the following is an example of a primary barrier in a microbiology laboratory?

  1. A hand-washing sink in the corridor
  2. A locked laboratory door
  3. A biological safety cabinet Correct answer.
  4. A fire extinguisher on the wall

Explanation

Primary barriers are containment devices that directly separate the worker from the infectious agent, such as biological safety cabinets and sealed centrifuge cups. Locked doors, sinks and fire extinguishers are secondary or facility-level controls.

Question 21Microscopy, Sterilisation and Culture Techniques

In a compound light microscope, which component is responsible for concentrating light onto the specimen and controlling the amount of light passing through it?

  1. Condenser with iris diaphragm Correct answer.
  2. Ocular lens
  3. Coarse adjustment knob
  4. Objective lens

Explanation

The substage condenser gathers light from the source and focuses it on the specimen, while the iris diaphragm regulates how much of that light enters the objective. Together they control illumination and contrast, which is essential for resolving stained bacterial smears.

Question 22Microscopy, Sterilisation and Culture Techniques

A student examines a Gram-stained smear with the 100x objective and a 10x eyepiece. What is the total magnification of the observed image?

  1. 110x
  2. 1000x Correct answer.
  3. 10000x
  4. 90x

Explanation

Total magnification is the product of the objective and eyepiece magnifications, so 100 multiplied by 10 gives 1000x. This oil-immersion setting is the standard working magnification for viewing individual bacterial cells.

Question 23Microscopy, Sterilisation and Culture Techniques

Immersion oil is placed between the slide and the 100x objective mainly because it:

  1. Provides nutrients that keep the smear moist
  2. Increases the numerical aperture by reducing light loss from refraction Correct answer.
  3. Prevents the objective from touching the glass slide
  4. Kills contaminating bacteria on the slide surface

Explanation

Immersion oil has a refractive index close to that of glass, so light rays that would otherwise be refracted away are captured by the objective. This raises the numerical aperture and improves resolution at high magnification.

Question 24Microscopy, Sterilisation and Culture Techniques

Which statement correctly describes the position of the condenser when examining an unstained wet mount for motile bacteria?

  1. The condenser should be removed from the light path entirely
  2. The condenser should be lowered and the diaphragm partly closed to increase contrast Correct answer.
  3. The condenser should be focused on the eyepiece rather than the specimen
  4. The condenser should be raised fully and the diaphragm fully open for maximum brightness

Explanation

Unstained cells have low contrast, so lowering the condenser and reducing the aperture increases contrast at the cost of some resolution. This trade-off makes faint, transparent bacteria easier to detect in a hanging-drop or wet-mount preparation.

Question 25Microscopy, Sterilisation and Culture Techniques

A microscope image appears sharp in the centre but blurred at the edges. Which optical defect is most likely responsible?

  1. Chromatic aberration
  2. Spherical aberration Correct answer.
  3. Insufficient magnification
  4. Poor resolution

Explanation

Spherical aberration arises because lens curvature causes peripheral rays to focus at a different point from central rays, blurring the field edges. It is corrected by using well-corrected objectives and by closing the diaphragm appropriately.

Question 26Microscopy, Sterilisation and Culture Techniques

Why is the oil-immersion objective always cleaned immediately after use?

  1. Oil left on the lens increases the numerical aperture permanently
  2. The oil causes the glass to dissolve slowly
  3. Residual oil can harden and damage the lens or trap dust and organisms Correct answer.
  4. The oil evaporates and leaves a corrosive salt residue

Explanation

Immersion oil dries to a sticky film that attracts dust, can scratch optical coatings and may harbour organisms, so it is wiped off with lens tissue and an appropriate solvent. Prompt cleaning preserves optical performance and prevents contamination between slides.

Question 27Microscopy, Sterilisation and Culture Techniques

In a Gram-stained smear, a decolourised Gram-negative cell appears pink because it:

  1. Contains a waxy mycolic acid layer that repels all dyes
  2. Loses the crystal violet–iodine complex and takes up the counterstain Correct answer.
  3. Absorbs safranin before the primary stain is applied
  4. Retains crystal violet in a thick peptidoglycan layer

Explanation

The thin peptidoglycan layer of Gram-negative cells does not retain the crystal violet–iodine complex during alcohol decolourisation. The colourless cell then takes up the pink safranin counterstain, giving the characteristic appearance.

Question 28Microscopy, Sterilisation and Culture Techniques

A heat-fixed bacterial smear is prepared before staining mainly to:

  1. Dissolve the cell wall to release pigments
  2. Sterilise the slide for safe handling
  3. Increase the cell size for easier viewing
  4. Attach the cells to the slide and improve dye uptake Correct answer.

Explanation

Gentle heating coagulates bacterial proteins so that cells adhere firmly to the slide and are not washed off during staining and rinsing. It also improves dye penetration and kills the organisms, reducing the risk of laboratory infection.

Question 29Microscopy, Sterilisation and Culture Techniques

Which autoclave cycle is appropriate for sterilising contaminated culture media and glassware?

  1. 121°C at 15 psi for 15 minutes Correct answer.
  2. 100°C at atmospheric pressure for 10 minutes
  3. 60°C at 5 psi for 60 minutes
  4. 180°C at 20 psi for 5 minutes

Explanation

Moist heat at 121°C under 15 psi for about 15 minutes denatures microbial proteins and reliably kills vegetative cells and endospores. These standard conditions are the basis of routine autoclave sterilisation in a microbiology laboratory.

Question 30Microscopy, Sterilisation and Culture Techniques

A tube of broth is autoclaved with its screw cap tightly sealed. What is the most likely consequence?

  1. The pH of the medium will rise sharply
  2. The broth will sterilise faster because pressure builds inside the tube
  3. The medium will remain unsterilised because steam cannot enter
  4. The cap may blow off or the tube may crack because pressure cannot equilibrate Correct answer.

Explanation

A tightly sealed vessel traps expanding air and steam during heating and cannot vent during pressure release. The resulting pressure difference can force the cap off or fracture the glass, so caps must be loosened before autoclaving.

Question 31Microscopy, Sterilisation and Culture Techniques

Culture media containing heat-labile substances such as serum or certain vitamins are best sterilised by:

  1. Dry heat at 160°C for two hours
  2. Autoclaving at 121°C for 15 minutes
  3. Boiling for 30 minutes
  4. Filtration through a sterile membrane filter Correct answer.

Explanation

Filtration removes bacteria physically without heating, so temperature-sensitive nutrients are not destroyed. Membrane filters of 0.22 µm pore size are typically used to produce sterile solutions that are then added aseptically to sterile base media.

Question 32Microscopy, Sterilisation and Culture Techniques

Which method is appropriate for sterilising a metal inoculating loop before use in the flame of a spirit lamp?

  1. Passing the loop briefly through the inner blue cone once
  2. Dipping the loop in alcohol and wiping it with cotton
  3. Rinsing the loop in sterile distilled water
  4. Holding the loop in the outer flame until it glows red Correct answer.

Explanation

Heating the loop in the flame until it is red hot incinerates any adhering organic matter and kills all microorganisms, including endospores. The loop must then be allowed to cool before touching a culture to avoid killing the organisms being transferred.

Question 33Microscopy, Sterilisation and Culture Techniques

A new batch of nutrient agar is prepared and poured into plates. Which control best verifies that the plates are sterile before use?

  1. Test the pH of the molten agar with indicator paper
  2. Observe the colour of the agar under normal light
  3. Weigh the plate to confirm the correct volume of medium
  4. Incubate one unopened plate from the batch and check for growth Correct answer.

Explanation

Incubating an uninoculated plate from the batch shows whether any contaminating organisms survived sterilisation or entered during pouring. Absence of growth after appropriate incubation confirms the batch is fit for use.

Question 34Microscopy, Sterilisation and Culture Techniques

In the streak-plate method, the purpose of flaming the loop between successive sectors is to:

  1. Progressively dilute the inoculum so that isolated colonies develop Correct answer.
  2. Cool the loop so that bacteria are not heat-shocked
  3. Prevent the agar from melting at the inoculation points
  4. Increase the nutrient concentration in later sectors

Explanation

Flaming the loop between sectors kills organisms carried on the wire, so each new sector receives fewer cells than the previous one. This serial dilution eventually yields well-separated single colonies derived from individual cells.

Question 35Microscopy, Sterilisation and Culture Techniques

A culture is incubated at 37°C in a candle jar. The main purpose of this arrangement is to:

  1. Maintain strict anaerobic conditions free of all oxygen
  2. Achieve a temperature above the normal incubation range
  3. Prevent evaporation of the culture medium
  4. Provide a microaerophilic atmosphere with reduced oxygen and added carbon dioxide Correct answer.

Explanation

A burning candle consumes oxygen and releases carbon dioxide, creating a reduced-oxygen, CO2-enriched atmosphere suitable for microaerophiles and capnophiles. It does not, however, remove all oxygen, so it is not a true anaerobic system.

Question 36Microscopy, Sterilisation and Culture Techniques

Which medium is described as selective because it inhibits Gram-positive organisms while allowing Gram-negative enteric bacilli to grow?

  1. MacConkey agar Correct answer.
  2. Nutrient agar
  3. Blood agar
  4. Tryptic soy agar

Explanation

MacConkey agar contains bile salts and crystal violet, which inhibit Gram-positive bacteria, and lactose with a pH indicator to detect lactose fermentation. It therefore selects for Gram-negative enteric organisms and differentiates them by colony colour.

Question 37Microscopy, Sterilisation and Culture Techniques

A bacterial culture is inoculated into a tube of solid agar medium using a straight needle, and growth appears only along the line of insertion. This pattern indicates that the organism is:

  1. Non-motile and growing only where inoculated Correct answer.
  2. A strict anaerobe growing at the surface
  3. Motile and able to spread through the medium
  4. Aerophilic and requiring surface growth

Explanation

In a stab culture, motile organisms migrate through the semisolid or solid medium and produce diffuse turbidity, whereas non-motile organisms grow only along the stab line. Growth confined to the needle track therefore indicates lack of motility.

Question 38Microscopy, Sterilisation and Culture Techniques

Which statement about the difference between disinfection and sterilisation is correct?

  1. Sterilisation eliminates all viable microorganisms including endospores Correct answer.
  2. Disinfection destroys all microbial forms including endospores
  3. Sterilisation applies only to living tissues
  4. Disinfection is always performed with heat

Explanation

Sterilisation is the complete destruction or removal of all viable microorganisms, including bacterial endospores and viruses. Disinfection reduces the number of pathogenic organisms on inanimate surfaces but does not necessarily eliminate endospores.

Question 39Microscopy, Sterilisation and Culture Techniques

An autoclave tape strip placed on a dressing pack turns dark after processing. What does this indicate?

  1. The autoclave pressure exceeded the set limit
  2. The pack is now sterile and ready for use without further checks
  3. The pack has been exposed to the required temperature-time conditions of the cycle Correct answer.
  4. All endospores in the pack have been destroyed

Explanation

Autoclave tape changes colour when exposed to the specified temperature, confirming that the pack passed through the cycle. It is an external indicator of process exposure, not proof of sterilisation, which requires biological or internal chemical indicators.

Question 40Microscopy, Sterilisation and Culture Techniques

A culture is incubated in a sealed anaerobic jar with a gas-generating sachet and a methylene blue indicator strip. The strip remains colourless after incubation. What does this show?

  1. The indicator has expired and is unreliable
  2. The temperature was too low for growth
  3. The jar leaked and oxygen entered
  4. The atmosphere is anaerobic, as required Correct answer.

Explanation

Methylene blue is blue in the presence of oxygen and becomes colourless when reduced in an anaerobic environment. A colourless strip therefore confirms that anaerobic conditions were achieved and maintained in the jar.

Question 41Microscopy, Sterilisation and Culture Techniques

In pour-plate technique, molten agar cooled to about 45–50°C is mixed with the inoculum before solidification. Why must the agar not be hotter?

  1. Hot agar would kill the bacterial cells being counted Correct answer.
  2. Hot agar would solidify too quickly to mix
  3. Hot agar would prevent condensation on the lid
  4. Hot agar would change the colour of the colonies

Explanation

Temperatures near boiling would kill or injure the organisms and give falsely low counts. Cooling to around 45–50°C keeps the agar molten long enough to mix while remaining safe for the bacteria.

Question 42Microscopy, Sterilisation and Culture Techniques

Which observation would most strongly suggest that a culture plate has been contaminated during handling rather than by the original inoculum?

  1. Several unrelated colony types appear on an uninoculated area of the plate Correct answer.
  2. Growth is absent after 48 hours of incubation
  3. Colonies of a single uniform type appear only along the streak lines
  4. Colonies are evenly distributed across all sectors

Explanation

Colonies with diverse morphology on areas that were never inoculated point to organisms introduced from the air, hands or bench. Such adventitious growth is a hallmark of poor aseptic technique rather than the intended culture.

Question 43Microscopy, Sterilisation and Culture Techniques

A student needs to transfer bacteria from a broth culture to a fresh agar slant. Which sequence reflects correct aseptic technique?

  1. Take the inoculum with a cold loop, then flame the loop after streaking
  2. Flame the loop and immediately plunge it into the broth while red hot
  3. Flame the loop, allow it to cool, take the inoculum, streak the slant, then flame the loop again Correct answer.
  4. Open both tubes and leave them open while streaking the slant

Explanation

The loop is sterilised before use, cooled to avoid killing the inoculum, used to transfer the bacteria, and then re-sterilised to prevent contamination of the environment and subsequent cultures. Tubes are flamed at the mouth and kept open only briefly during transfer.

Question 44Microscopy, Sterilisation and Culture Techniques

A laboratory receives a heat-sensitive antibiotic solution and a batch of glass Petri dishes. Which pairing of sterilisation methods is most appropriate?

  1. Antibiotic solution by boiling; glass dishes by flaming
  2. Antibiotic solution by dry heat; glass dishes by autoclaving
  3. Antibiotic solution by filtration; glass dishes by dry heat Correct answer.
  4. Antibiotic solution by autoclaving; glass dishes by filtration

Explanation

Filtration sterilises the antibiotic solution without heat, preserving its activity, while dry heat at about 160–170°C for two hours reliably sterilises glassware. Each method is matched to the heat sensitivity of the material.

Question 45Morphology, Physiology and Growth of Microorganisms

A bacterial cell is observed under the microscope and its chromosome is found lying free in the cytoplasm without a surrounding nuclear envelope. This arrangement is best described as which type of cell organization?

  1. Eukaryotic
  2. Viral
  3. Protozoan
  4. Prokaryotic Correct answer.

Explanation

Prokaryotes lack a membrane-bound nucleus, so their single circular chromosome lies directly in the cytoplasm. Eukaryotes enclose their DNA within a nuclear envelope. Viruses are not cells at all, and protozoans are eukaryotic organisms.

Question 46Morphology, Physiology and Growth of Microorganisms

Which structure gives a bacterial cell its characteristic shape and protects it from osmotic lysis?

  1. Cytoplasmic membrane
  2. Flagellum
  3. Capsule
  4. Cell wall Correct answer.

Explanation

The peptidoglycan cell wall is a rigid mesh that maintains cell shape and resists internal osmotic pressure. Without it, bacteria lyse in hypotonic surroundings. The capsule, membrane and flagellum serve other functions.

Question 47Morphology, Physiology and Growth of Microorganisms

A Gram-stained smear shows purple-blue cells after decolourization and counterstaining. What does this result indicate about the cell wall?

  1. An outer membrane dissolved the crystal violet-iodine complex
  2. A thin peptidoglycan layer allowed the primary stain to wash out
  3. Absence of peptidoglycan caused retention of the counterstain
  4. A thick peptidoglycan layer retained the crystal violet-iodine complex Correct answer.

Explanation

Gram-positive cells have a thick peptidoglycan layer that traps the crystal violet-iodine complex during alcohol decolourization, so they remain purple. Gram-negative cells lose the complex through their thin wall and appear pink after safranin counterstaining.

Question 48Morphology, Physiology and Growth of Microorganisms

A culture is incubated in a sealed jar with a chemical sachet that removes oxygen and generates carbon dioxide. Which nutritional class of organism is being selected for?

  1. Microaerophile
  2. Facultative anaerobe
  3. Obligate anaerobe Correct answer.
  4. Obligate aerobe

Explanation

An anaerobic jar removes oxygen, creating an environment suitable only for organisms that grow without oxygen, such as obligate anaerobes. Aerobes and microaerophiles require some oxygen, and facultative anaerobes grow best with oxygen present.

Question 49Morphology, Physiology and Growth of Microorganisms

In a bacterial growth curve, which phase shows the highest rate of cell division with no net increase in total cell mass?

  1. Stationary phase
  2. Death phase
  3. Log phase Correct answer.
  4. Lag phase

Explanation

During the exponential or log phase, cells divide at a constant maximum rate and the population doubles regularly. Nutrients are still plentiful and waste products have not yet accumulated to inhibitory levels.

Question 50Morphology, Physiology and Growth of Microorganisms

A culture medium contains peptone, beef extract and sodium chloride but no agar, and it is dispensed into tubes. How is this medium classified?

  1. Liquid broth medium Correct answer.
  2. Selective medium
  3. Semi-solid medium
  4. Solid medium

Explanation

A medium without a solidifying agent such as agar remains liquid and is called a broth. It supports growth throughout the volume and is used for propagation and biochemical tests. Solid and semi-solid media require agar or gelatin.